1. Field of the Invention
The present invention pertains to an IC test apparatus (IC tester) and a device power supply used in the test apparatus, and in further detail, to a power supply referred to as an SMU (Source Measure Unit) or DPS (Device Power Supply) that applies voltage and current to and performs measurement on an IC, and a test apparatus that uses the power supply.
2. Discussion of the Background Art
The SMU as shown in Japanese Laid-Open Patent No. Sho 58-121812 has been used in the past as a device power supply for the IC of an IC test apparatus, such as IC testers and DC parametric test systems, etc. By using the SMU, it has been possible to perform function tests on applying voltage to the terminal of the IC while limiting current, or to measure current with applying voltage to the IC test apparatus, with a short settling time.
As shown in FIG. 1, device power supply 10 having a negative feedback amplifier according to the prior art, for instance, includes input resistor R214, amplifier A116, current detection resistor R318, inductances L120, L221, and L325, load impedance Z122, buffer B126, feedback resistor R128, capacitor C130, input terminal 12 and its voltage VIn, force terminal 52 on the high output-side of the apparatus, sense terminal 54 on the high output-side of the apparatus, and DUT terminal 24 and its voltage Vout. Load impedance Z122 here includes impedances such as the capacitive components of the termination resistor and by-pass capacitor, capacitive and/or inductive components of filter circuit for eliminating noise, and the impedance of the device under test (DUT). Buffer B126 monitors and buffers the voltage Vout of DUT terminal 24. Inductance L120 is an inductance due to the extended cable from one end of R318, connected to inductance L120, in the cage of the IC test apparatus on which device power supply 10 is set up, to force terminal 52 on the high output-side of the test head where the DUT is put on. And similarly, inductance L325 is an inductance due to the extended cable from the sense terminal on the high output-side of the test head, to buffer B126 inside the cage. L221 is the inductive component of the filter that has been connected for noise elimination. The output of amplifier A116 is connected to force terminal 52 on the high output-side with R318 and L120 in between. Sense terminal 54 on the high output-side is connected to the inverting input of A116 with L325, B126, and R128 in between and makes up part of the negative feedback loop. Force terminals and sense terminals are similarly set up on the low output-side of device power supply 10 in the case of full Kelvin connection (four-terminal connection), but in order to simplify the description, only the grounded terminals are shown in FIG. 1 and the details are omitted.
Voltage Vin in conjunction with the setting voltage is applied to input terminal 12. Input terminal 12 is connected to the inverting input of amplifier A116 and therefore, a feedback amplifier is made by the path A1-R3-L1-terminal 52-L2-terminal 24-terminal 54-L3-B1-R1-R2 and the voltage represented by Vout=xe2x88x92R1/R2*Vin is output to DUT terminal 24.
Suppose that there is no inductance L1, L2, or L3 in the cable and the filter circuit as an ideal case in order to consider the transfer characteristics of the entire loop. in the case, it is supposed that load Z1 is consisted of pure resistive component. There is no phase delay and gain is a value between 0 and 1 in the transfer characteristics from R3 and Z1. Since a circuit consisting of L3, B1, R1, R2, C1 and A1 make an integrator, the phase delay of their transfer characteristics is 90 degrees. Consequently, the phase delay of the overall transfer characteristics is a maximum of 90 degrees and therefore, the transfer characteristics of the total feedback loop are stable, regardless of the gain of the integrator.
Nevertheless, in actual measurements, if a large current of one ampere (A) or greater is applied to the DUT, very small resistance is used in order to reduce the voltage drop at resistor R318. Sometimes, inductance L1 and L3 due to the extended cable become to be relatively large. An L-C filter can be added in order to reduce various types of noise and therefore, inductance of the L-C filter is added to inductance L221. Accordingly, the capacitive component of the L-C filter (referred to as Cz) and the capacitive component of the by-pass capacitor are included in Z122.
In this case, R3-L1-L2-Z1 becomes the secondary resonant circuit of L-R-C. Moreover, as previously mentioned, since R3 is small, the quality factor of the secondary resonant circuit is high. As a result, the phase delay of the transfer characteristics from the output of amplifier A116 to DUT terminal 24 has a maximum angle of 180 degrees (when R3 is 0 xcexa9.). However, in normal cases, R3 is not at 0 xcexa9 and therefore, it becomes 140 degrees for instance.
Next, the transfer characteristics from DUT terminal 24 to the output of buffer B126 will be considered. The effects of inductance L325 can be disregarded because the input impedance of buffer B126 is high and therefore, as in the case where there is no inductance L325, the transfer characteristics become a gain of 1 and a phase delay of 0 degrees. The transfer characteristics from the output of buffer B126 to the output of amplifier A116 become integration characteristics and the phase delay becomes 90 degrees.
As a result, the phase delay angle of overall transfer characteristics is a maximum of 270 degrees (when R3 is 0 xcexa9). And if the gain of the overall transfer characteristics become 0 dB or higher at the frequency where phase delay is over 180 degrees, oscillation will occur. However, in most cases, R3 is not 0 xcexa9 and the phase delay becomes smaller than 180 degrees, therefore, oscillation does not occur, but ringing can occur because of the less phase margin.
As previously mentioned, there are problems with conventional device power supplies in that oscillation or ringing readily occurs, when the capacitive component of load Z1 is large. Once oscillation or ringing have occurred, a higher voltage than the maximum allowable voltage can be applied to the power source terminals of the device, then, the device itself will be damaged. The device can also break down. Moreover, as a result of the ringing, etc., a good device can also be evaluated as a defective one.
Furthermore, the power current has increased with the recent increase in speed and reduction in operating voltage of the IC and therefore, resistance R318 tends to be lower. Therefore, it becomes necessary to prevent oscillation and ringing of device power supplies.
In light of these problems of the prior art, the present invention provides a device power supply and IC test apparatus with which there is little oscillation, even if the load capacitive component is large, with the short stabilizing time remaining uncompromised to the utmost during IC tests.
The present invention presents a device power supply, having an amplifier, a high output-side force terminal connected to the output of the amplifier, a high output-side sense terminal, and a first feedback circuit from the high output-side sense terminal to the input of the amplifier, where a first low-pass filter is placed between the amplifier output and the first feedback circuit. Moreover, there is a first inductance between the first low-pass filter and the high output-side force terminal. By means of this type of structure, oscillations rarely occur, even under a load with a large volume component.
Moreover, the first low-pass filter can have a capacitor that connects the amplifier output and the first feedback circuit and a resistor inserted in series between the capacitor connection terminal of the first feedback circuit and the high output-side sense terminal. And it can have a resistor and capacitor connected in series, which connect the amplifier output and the first feedback circuit and a resistor connected in series that connect the connection terminal of the capacitor for the first feedback circuit and the high output-side sense terminal.
Furthermore, in the case of full Kelvin connection, the first feedback circuit is connected to the inverting input of the amplifier. The device power supply further has a low output-side force terminal and a low output-side sense terminal. The low output-side force terminal is grounded with a second inductance in between, the low output-side sense terminal is connected to the non-inverting input of the amplifier with a second feedback circuit in between, and the terminal on the grounded side of the second feedback circuit and the second inductance have a second low-pass filter.
Moreover, the IC test apparatus of the present invention comprises an amplifier, a load impedance connected to the output of the amplifier, and a feedback circuit from the load impedance to the input of the amplifier, and there is a first low-pass filter between the amplifier output and feedback circuit.
Moreover, the feedback circuit of this IC test apparatus is connected to the inverting input of the amplifier, and the terminal where the load impedance is not connected to the output of the amplifier can be connected to the inverting input of the amplifier with the second low-pass filter and the second inductance in between.
Furthermore, low-pass filters can be implemented by a primary low-pass filter, or a higher order, such as a secondary or higher, low-pass filter.
As previously mentioned, when the present invention is used, it is possible to present a device power supply for an IC test apparatus with which there is hardly any oscillation, even if the capacitive component of the load is large or if various power source filters have been introduced. As a result, a device power supply can be presented with which there is hardly any oscillation, the settling time is short, and the noise is low.